electrosynthesis of hydrogen peroxide
TRANSCRIPT
INTRODUCTIONHydrogen peroxide (H2O2) is a strong oxidizer, con-
sidered environmentally friendly and used for blea-
ching, chemical synthesis, waste water treatment and
disinfection in various industries. Decentralised H2O2
production via electrochemistry minimises shipment,
handling and storage and avoids the use of hydrogen,
solvents and complex processing for catalyst recycle
in case of the anthraquinone autoxidation process.
H2O2 electrosynthesis is usually performed in strong
alkaline solutions [1] in which H2O2 is least stable [2].
Furthermore, high H2O2 concentrations can have a
negative effect by H2O2 reduction to H2O. Utilisa-
tion of electrodialysis principles (Figure 1) proved
to be effective in removing H2O2 as HO2- from the
catholyte [3], which is however limited by the stabi-
lity of the anion exchange membrane (AEM) [4]. In
addition, potential applications are limited due to
salts and/or acids used in the centre compartment or
require complex H2O2 purification methods.
OBJECTIVEThe objective of our research is to achieve H2O2 pro-
duction using stable anion exchange membranes,
mathematical modeling of the process and subse-
quently the development of an electrolyser concept
towards salt (and acid) free production of H2O2.
RESULTSA gas diffusion electrode (GDE) consisting of a mix-
ture of PTFE, graphite and acetylene black is used as
cathode and iridium MMO coated titanium as anode.
A minimum energy usage of 2.7 and 3.2 kWh/kg
H2O2 is derived from polarisation curves (Figure 2)
at 0.5 and 3.0 kA/m2, respectively. Ion exchange mem-
branes with an assumed area resistance of 4 Ω·cm2
increases energy usage to an acceptable 3.0 and 5.2
kWh/kg H2O2.
The electrode kinetics are fitted to the Butler-
Volmer equation and combined with diffusional
transport of oxygen (GDE).
The electrode kinetics, mass transport (ext. Nernst
Planck equation) and equilibrium chemistry are com-
bined in a non-steady state model enabling simulation
of batch-recycle operation. H2O2 production in the
catholyte and centre compartment and the cell vol-
tage are in line with the mathematical model (Figure
3). 100 g/L H2O2 is achieved at high current effici-
ency, ~85%, with no deterioration noticed of the AEM
(Neosepta AHA). Obtained water transport num-
bers of 2.9 and 3.5 mol/F for CEM and AEM respecti-
vely indicates a max. concentration of 120 g/l H2O2.
The initial energy usage of 16 kWh/kg H2O2 (mainly
caused by iR drop in the aqueous electrolytes) incre-
ased to 40 kWh/kg H2O2 due to dilution effects in the
centre compartment and illustrates the necessity of
salt or acid addition which is undesired from a cost
and processing perspective. Initial trials with PFSA
based solid polymer electrolyte (SPE) in the centre
compartment are successful and enables continuous
production of ~70 g/L H2O2 for almost 100 hours at
64% current efficiency. The H2O2 product contains
only 0.05 mM K+ and 4 mM SO42- probably caused
by non-ideality of ion exchange membranes. Design
of the SPE appears crucial as energy usages of 11
to 80 kWh/kg H2O2 are obtained with various SPE
structures.
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Electrosynthesis of hydrogen peroxide
Figure 1: Schematic representation of the used principle for electrosynthesis of H2O2
Figure 2: Polarisation curves of GDE (3.1 cm2) in 0.1 M KOH (black line) with pure O2
supplied at the backside and Ir-MMO (3.0 cm2) in 0.1 M H2SO4 (red line) at 1 mV/s
and room temperature (~20°C).
Figure 4: H2O2 concentrations during electrolysis at 1 A using a 10 cm2 plate-
and-frame electrochemical cell in a batch-recycle set-up with continuous
removal of H2O2, centre compartment filled with Nafion NR50 beads and initially
demineralised water.
Figure 3: H2O2 concentrations and cell voltage during electrolysis at 5 A using a
100 cm2 plate-and-frame electrochemical cell equipped with Nafion 117 and
Neosepta AHA membranes. 2 litres 0.1 M H2SO4, 2 litres 0.1 M KOH catholyte and
0.3 litres 0.1 M K2SO4 were circulated through the anolyte, catholyte and centre
compartments, respectively. Pure oxygen was fed to the backside of the GDE.
The solid lines were obtained using the mathematic model.
REFERENCES
[1] D. Pletcher, Watts New 4 (1999), 1-7.
[2] J.A. Navarro et al., Chem. Soc., Faraday Trans. I,
80 (1984), 249-253.
[3] C. Kuehn et al., US4357217 (1981).
[4] C. Kuehn et al., J. Electrochem. Soc., 130 (1983), 1117-1119.
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CONCLUSION & OUTLOOK 1. A concentration of 100 g/L H2O2 is
achieved experimentally while maintaining
the structural and functional integrity of
the membranes.
2. The developed mathematical model describes
essential parameters of the electrochemical
process satisfactory.
3. Incorporation of SPE between both ion
exchange membranes enables direct
H2O2 electrosynthesis with low salt and
acid content.
Next steps in our research include attempts to
reduce energy usage by minimisation of iR drop
through SPE and cell design optimisation.
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